Blades and White, U.S. Pat. No. 3,081,519 describe flash-spinning plexifilamentary film-fibril strands from fiber-forming polymers. A solution of the polymer in a liquid, which is a non-solvent for the polymer at or below its normal boiling point, is extruded at a temperature above the normal boiling point of the liquid and at autogenous or higher pressure into a medium of lower temperature and substantially lower pressure. This flash spinning causes the liquid to vaporize and thereby cool the extrudate which forms a plexifilamentary film-fibril strand of the polymer. According to Blades and White, the following liquids are useful in the flash-spinning process: aromatic hydrocarbons such as benzene, toluene, etc.; aliphatic hydrocarbons such as butane, pentane, hexane, heptane, octane, and their isomers and homologs; alicyclic hydrocarbons such as cyclohexane; unsaturated hydrocarbons; halogenated hydrocarbons such as methylene chloride, carbon tetrachloride, chloroform, ethyl chloride, methyl chloride; alcohols; esters; ethers; ketones; nitriles; amides; fluorocarbons; sulfur dioxide; carbon disulfide; nitromethane; water; and mixtures of the above liquids. The patent further states that the flash-spinning solution additionally may contain a dissolved gas, such as nitrogen, carbon dioxide, helium, hydrogen, methane, propane, butane, ethylene, propylene, butene, etc. Preferred for improving plexifilament fibrillation are the less soluble gases, i.e., those that dissolve to a less than 7% concentration in the polymer solution under the spinning conditions.
Blades and White state that polymers which may be flash spun include those synthetic filament-forming polymers or polymer mixtures which are capable of having appreciable crystallinity and a high rate of crystallization. A preferred class of polymers is the crystalline, non-polar group consisting mainly of crystalline polyhydrocarbons, such as polyethylene and polypropylene.
U.S. Pat. No. 3,169,899 lists polyester, polyoxymethylene, polyacrylonitrile, polyamide, polyvinyl chloride, etc. as other polymers that may be flash spun. Still other polymers mentioned in the patent are flash spun as mixtures with polyethylene, including ethylene vinyl alcohol, polyvinyl chloride, polyurethane, etc. Example 18 of U.S. Pat. No. 3,169,899 illustrates flash spinning from methylene chloride of a mixture of polyethylene and ethylene vinyl alcohol in which polyethylene is the predominant component of the polymer mixture.
Flash spun polyethylene products have achieved considerable commercial success. "TYVEK.RTM." spunbonded olefin is a spunbonded polyethylene sheet product sold by E. I. du Pont de Nemours and Company. These sheets are used in the construction and packaging industries. "TYVEK.RTM." spunbonded olefin is also used in protective apparel since the flash spun product provides a good barrier to particulate penetration. However, the hydrophobic nature of polyethylene results in a garment which tends to be uncomfortable during hot, humid weather. A more hydrophilic flash spun product is clearly desirable for garment and some other end uses. Additionally, flash spinning of any of the polymers would preferably be achieved from an environmentally safe, non-toxic solvent.
Trichlorofluoromethane ("FREON.RTM.-11) has been a very useful solvent for commercial manufacture of plexifilamentary film-fibril strands of polyethylene. However, the escape of such a halocarbon into the atmosphere has been implicated as a serious source of depletion of the earth's ozone. A general discussion of the ozone-depletion problem is presented, for example by P. S. Zurer, "Search Intensifies for Alternatives to Ozone-Depleting Halocarbons", Chemical & Engineering News, pages 17-20 (Feb. 8, 1988). The substitution of environmentally safe solvents for trichlorofluoromethane in a commercial flash spinning process should minimize the ozone depletion problem.
There now has been discovered in accordance with this invention, flash spun polymer products desirable for uses such as garments, construction and packaging, which are flash spun from an environmentally acceptable mixture comprising carbon dioxide and water.